The present disclosure is generally related to curable inks, particularly curable phase change white inks, and their use in methods for forming images, particularly their use in ink jet printing. One embodiment disclosed herein comprises a radiation curable phase change ink comprising a white colorant; a colorant dispersant; and an ink vehicle comprising at least one curable monomer; at least one gellant; at least one photoinitiator; optionally at least one stabilizer; and optionally at least one wax.
Phase change inks for color printing typically comprise a phase change ink carrier composition which is combined with a phase change ink compatible colorant. In a specific embodiment, a series of colored phase change inks can be formed by combining ink carrier compositions with compatible subtractive primary colorants. The subtractive primary colored phase change inks can comprise four component dyes, namely, cyan, magenta, yellow and black, although the inks are not limited to these four colors. These subtractive primary colored inks can be formed by using a single dye or a mixture of dyes. For example, magenta can be obtained by using a mixture of Solvent Red Dyes or a composite black can be obtained by mixing several dyes. U.S. Pat. No. 4,889,560, U.S. Pat. No. 4,889,761, and U.S. Pat. No. 5,372,852, the disclosures of each of which are totally incorporated herein by reference, teach that the subtractive primary colorants employed can comprise dyes from the classes of Color Index (C.I.) Solvent Dyes, Disperse Dyes, modified Acid and Direct Dyes, and Basic Dyes. The colorants can also include pigments, as disclosed in, for example, U.S. Pat. No. 5,221,335, the disclosure of which is totally incorporated herein by reference. U.S. Pat. No. 5,621,022, the disclosure of which is totally incorporated herein by reference, discloses the use of a specific class of polymeric dyes in phase change ink compositions.
Phase change inks have also been used for applications such as postal marking, industrial marking, and labeling.
Phase change inks are desirable for ink jet printers because they remain in a solid phase at room temperature during shipping, long term storage, and the like. In addition, the problems associated with nozzle clogging as a result of ink evaporation with liquid ink jet inks are largely eliminated, thereby improving the reliability of the ink jet printing. Further, in phase change ink jet printers wherein the ink droplets are applied directly onto the final recording substrate (for example, paper, transparency material, and the like), the droplets solidify immediately upon contact with the substrate, so that migration of ink along the printing medium is prevented and dot quality is improved.
Compositions suitable for use as phase change ink carrier compositions are known. Some representative examples of references disclosing such materials include U.S. Pat. No. 3,653,932, U.S. Pat. No. 4,390,369, U.S. Pat. No. 4,484,948, U.S. Pat. No. 4,684,956, U.S. Pat. No. 4,851,045, U.S. Pat. No. 4,889,560, U.S. Pat. No. 5,006,170, U.S. Pat. No. 5,151,120, U.S. Pat. No. 5,372,852, U.S. Pat. No. 5,496,879, European Patent Publication 0187352, European Patent Publication 0206286, German Patent Publication DE 4205636AL, German Patent Publication DE 4205713AL, and PCT Patent Application WO 94/04619, the disclosures of each of which are totally incorporated herein by reference. Suitable carrier materials can include paraffins, microcrystalline waxes, polyethylene waxes, ester waxes, fatty acids and other waxy materials, fatty amide containing materials, sulfonamide materials, resinous materials made from different natural sources (tall oil rosins and rosin esters, for example), and many synthetic resins, oligomers, polymers, and copolymers.
Ink jetting devices are known in the art, and thus extensive description of such devices is not required herein. As described in U.S. Pat. No. 6,547,380, incorporated herein by reference, ink jet printing systems generally are of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field that adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
There are at least three types of drop-on-demand ink jet systems. One type of drop-on-demand system is a piezoelectric device that has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. Another type of drop-on-demand system is known as acoustic ink printing. As is known, an acoustic beam exerts a radiation pressure against objects upon which it impinges. Thus, when an acoustic beam impinges on a free surface (i.e., liquid/air interface) of a pool of liquid from beneath, the radiation pressure which it exerts against the surface of the pool may reach a sufficiently high level to release individual droplets of liquid from the pool, despite the restraining force of surface tension. Focusing the beam on or near the surface of the pool intensifies the radiation pressure it exerts for a given amount of input power. Still another type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink vehicle (usually water) in the immediate vicinity to vaporize almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands.
In a typical design of a piezoelectric ink jet device utilizing phase change inks printing directly on a substrate or on an intermediate transfer member, such as the one described in U.S. Pat. No. 5,372,852, incorporated herein by reference, the image is applied by jetting appropriately colored inks during four to eighteen rotations (incremental movements) of a substrate (an image receiving member or intermediate transfer member) with respect to the ink jetting head, i.e., there is a small translation of the printhead with respect to the substrate in between each rotation. This approach simplifies the printhead design, and the small movements ensure good droplet registration. At the jet operating temperature, droplets of liquid ink are ejected from the printing device and, when the ink droplets contact the surface of the recording substrate, either directly or via an intermediate heated transfer belt or drum, they quickly solidify to form a predetermined pattern of solidified ink drops.
Hot melt inks typically used with ink jet printers have a wax based ink vehicle, e.g., a crystalline wax. Such solid ink jet inks provide vivid color images. In typical systems, these crystalline wax inks partially cool on an intermediate transfer member and are then pressed into the image receiving medium such as paper. Transfuse spreads the image droplet, providing a richer color and lower pile height. The low flow of the solid ink also prevents show through on the paper.
In these systems, the crystalline wax inks are jetted onto a transfer member, for example, an aluminum drum, at temperatures of approximately 130° C. to about 140° C. The wax based inks are heated to such high temperatures to decrease their viscosity for efficient and proper jetting onto the transfer member. The transfer member is at approximately 60° C., so that the wax will cool sufficiently to solidify or crystallize. As the transfer member rolls over the recording medium, e.g., paper, the image comprised of wax based ink is pressed into the paper.
Radiation curable inks generally comprise a curable monomer, a curable wax, a colorant, and a radiation activated initiator, specifically a photoinitiator, that initiates polymerization of curable components of the ink, specifically of the curable monomer and the curable wax. White radiation curable inks are considered a high value added material for printers. White inks are required for certain graphic and labeling applications. They are most typically used on transparent and dark substrates to compliment or provide a background to text or graphics. White text on smoke colored plastic is particularly striking. White colorants are pigments, which can comprise a variety of materials, most typically titanium dioxide. A general description of titanium dioxide properties can be found, for example, in Kirk-Othmer Encyclopedia of Chemical Technology, 4th Ed., (John Wiley & Sons, NY), Vol. 24, pages 239-240, which is hereby incorporated by reference herein. Because white is produced by light scattering, a pigment having a volume average particle size of from about 200 to about 300 nanometers is generally selected to provide a good white. As the particle size in a dispersion decreases below this range, the dispersion selectively scatters blue light and appears bluish white. As the particle size decreases further, it will not scatter visible light and will become transparent. For example, smaller nanometer sized titanium dioxide pigments are known, but they are colorless. In embodiments herein, an effective white, light scattering is achieved by selecting optimally sized titanium dioxide particles and increased pigment loadings of approximately about 5 to about 30 volume percent based upon the total volume of the cured ink layer. In further embodiments herein, an effective white, light scattering is achieved by selecting optimally sized titanium dioxide particles and increased pigment loadings of approximately about 10 volume percent based upon the total volume of the cured ink layer. The weight percent of a 10 volume percent loading is effectively about 40 percent due to the high density of the titanium dioxide particles relative to the curable vehicle. The combination of dense dispersions, for example, from about 2 to about 4 grams of pigment per milliliter of pigment and dispersion, relatively large size, and high loadings of pigment make it particularly challenging to design a storage stable and jettable white curable ink. Additionally curing is often difficult to accomplish due to high pigment loading and high scattering light required for initiation.
While known compositions and processes are suitable for their intended purposes, a need remains for phase change inks that are suitable for ink jet printing processes, such as piezoelectric ink jet processes and the like. In addition, a need remains for phase change inks that are suitable for acoustic ink jet printing processes. Further, a need remains for improved curable phase change inks, for example for radiation curable phase change white inks with improved storage stability, low Newtonian viscosity, and that cure well.
The appropriate components and process aspects of the each of the foregoing may be selected for the present disclosure in embodiments thereof.